There is a known omnidirectional imaging system which includes multiple wide-angle lenses such as fisheye lens or super wide-angle lens to capture an image in omnidirections at once. It is configured to project images from the lenses onto a sensor surface and combine the images through image processing to thereby generate an omnidirectional image. For example, by use of two wide-angle lenses with angle of view of over 180 degrees, omnidirectional images can be generated. In the image processing a partial image captured by each lens system is subjected to distortion correction and projection conversion on the basis of a certain projection model with a distortion from an ideal model taken into account. Then, the partial images are connected on the basis of an overlapping portion of the partial images to form a single omnidirectional image.
In related art an exposure correction technique of a digital camera to acquire a proper exposure from a captured image is known. For instance, Japanese Patent Application Publication No. 2007-329555 discloses an imaging system including multiple imaging units arranged to have an overlapping imaging area to extract an overlapping area from each of the images captured by the imaging units. It is configured to adjust at least one of the exposure and white balance of the imaging units according to each image of the extracted overlapping areas to reduce a difference in the brightness or color of the captured images, for the purpose of abating workloads of post-processing such as synthesis.
However, it is difficult for the omnidirectional imaging system to acquire a proper exposure by such a related-art exposure correction technique because the optical conditions or photographic circumstances of the imaging units thereof differ. The related art disclosed in the above document only concerns an overlapping area so that it cannot obtain appropriate exposure correction values under an unbalanced exposure condition if the overlapping area is small relative to the entire image. In particular, since the imaging area of the omnidirectional imaging system is omnidirectional, a high-brightness subject as the sun is often captured on a sensor, which may cause a flare and an increase in image offset value. A proper exposure can be obtained for each sensor, however, it may cause a difference in brightness between the connecting portions of the images and impair the quality of an omnidirectional image.